A permeable asphalt trail will replace a muddy pathway for pedestrians, and give slow bicyclists an alternative to cringing alongside the passing cars on Lake Washington Boulevard East.

Jack Collins, chairman of the Botanical Garden Committee, said the state Department of Transportation will spend $7.8 million on Arboretum improvements — mainly the trail — and perhaps $12 million later to build a road roundabout, along with wetland and creek restorations.

The ghost ramps have loomed as a vestige of mid-20th century mania for the automobile. Leaders seriously considered multiple bridges crossing Puget Sound, as well as an east-west Bay Freeway linking Interstate 5 to Highway 99 north of downtown Seattle.

The state and the city proposed to build an R.H. Thomson Expressway connecting the Central District to Northeast Seattle, including ramps that were part of the original 520 bridge when it opened in 1963. After citizens protested, the Seattle City Council killed the expressway plan in 1971, making Seattle one of many cities where freeway revolts left unfinished ramps behind.

Generations of young people have leapt from an unused 38-foot-high overpass into an approximately 12-foot-deep fen where creek water meets Lake Washington, brushing their feet against the silty bottom. At least one jumper died, in 1986, after landing on an aluminum canoe.

Skateboarders have built quarter-pipe ramps on the deck spans. Homeless people have slept beneath them.

A short walk east is a popular shoreline, built on a former landfill that’s been recolonized by nature. Swimmers cross the waterway and climb aboard another unused ramp. Summer visitors have enjoyed picnics, rope swings, prayer meet-ups, guitar singalongs, clothing-optional sunbathing, canine games of fetch and even the occasional tryst.

Ramp removal accounts for about $15 million of the $382 million “Lake to Land” phase of Highway 520, in which new westbound lanes will connect from the floating part of the bridge to the shore at Montlake Boulevard East.

But it’s unclear if the ramps will be torn out right away in 2014 or later in the construction contract, said Julie Meredith, DOT’s project director on 520.

Their disappearance will open a large swath that’s now bereft of plants and covered by trash.

“See all of this? It all goes away!” said Paige Miller, Arboretum Foundation executive director, waving her arms at the concrete leviathan.

Other ramps connect Lake Washington Boulevard to the Highway 520 toll bridge, and will remain in use by drivers, until the entire Seattle landing is replaced, maybe by the early 2020s.

These will not be rebuilt. Instead, drivers will take 24th Avenue East and enter 520 at an expanded Montlake Boulevard interchange.

And it is hoped, said Miller, this means more quiet and less traffic inside the Arboretum.

Material from Historylink.org is included in this story. Seattle Times news researcher Miyoko Wolf contributed to this report. Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com.